Corrective surgery is available for most types of congenital heart defects. Many children who undergo open heart surgery develop postoperative arrhythmias that are sudden, unexpected and life-threatening. The incidence of sudden death in patients who have had repairs of tetralogy of Fallot and transposition of the great arteries exceeds 5%. The goals of this study will be to use the electrophysiologic study in these patients to 1) identify the specific types of rhythm disturbances associated with a particular lesion, 2) identify which electrophysiologic factors may be used to identify patients at high risk for developing serious arrhythmias, 3) to investigate and identify the electrophysiologic mechanisms of specific postoperative arrhythmias, and 4) to develop a logical and effective approach to the choice of a specific antiarrhythmic regimen by determining the electrophysiologic effects and effectiveness of drugs on specific arrhythmias in children. The current empiric approaches to postoperative arrhythmias have not decreased the incidence of sudden death. It is hoped that a greater understanding of the development of these arrhythmias including the use of an objective method to identify high risk patients, along with knowledge of mechanisms and objective information regarding the electrophysiologic effects of a specific therapy would provide for a more rational and effective approach to the management of arrhythmias in these children. The electrophysiologic methods used in this study will include the use of programmed electrical stimulation to evaluate electrophysiologic function of the sinus and AV nodes as well as conduction and refactoriness in the atrium, AV node and ventricle. The vulnerability of the atria and ventricles to develop arrhythmias will be assessed using the stimulation techniques of rapid pacing and premature extrastimuli. Endocardial catheter mapping techniques will be used to assess conduction and activation within the atria and ventricles. Numerous electrophysiologic parameters will be carefully evaluated to determine the most likely mechanism responsible for spontaneous or induced arrhythmias. Electrophysiologic testing in conjunction with drug administration (both oral and intravenous) will be used to evaluate mechanisms as well as to determine effective antiarrhythmic regimens.